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Introduction

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Passamaquoddy Bay is a large (575 km2) embayment located in southwestern New Brunswick, Canada.

Fig. 1

Fig 1. Passamaquoddy Bay location.

One of its more remarkable features is the multitude of depressions or “pockmarks’ on its floor (Pecore and Fader, 1990). Formative mechanisms may include:
  1. Methane ebullition entraining sediments which are dispersed by local currents;
  2. Seismic activity which resuspends sediment following the discharge of subterranean liquids (including methane);
  3. Meteorite impact;
  4. Glacial erosion and tectonic activity leading to the formation of hill – hole pairs;
  5. Thermogenic gas that accumulates behind a gas hydrate seal and is explosively released as a glacial period ends.
The Passamaquoddy Bay area is seismically active, e.g. the Oak Bay Fault that runs in the Saint Croix estuary (Fader, 1988), and might provide the energy needed for the sudden release of sedimentary gas. As far as we are aware the age of any of the pockmarks has not yet been determined.

Fig. 2

Fig 2. Areas of occurrence of pockmarks, eyed pockmarks and buried pockmarks in Passamaquoddy Bay interpreted from sidescan sonograms, echograms and seismic reflection profiles (Pecore and Fader, 1990).

According to Fader (1990) “…pockmarks have been found in many continental shelf environments of the world and their role as foci of intense chemosynthetic activity has attracted considerable attention”. There are estimated to be approximately 11,000 pockmarks in Passamaquoddy Bay covering an area of approximately 87 km2. They occur on Holocene clay, the uppermost unit of the seafloor in two patches separated by a ridge of till-covered bedrock, (Pecore and Fader, 1990). Some are aligned in parallel rows while others occur in clusters and, in some cases, smaller pockmarks occur inside larger ones. Their diameter ranges from 1 to 300 m with an average diameter of 29m and they occur in water depths of 20-40m. While their average depth is 3.5m, the range is substantial, the deepest being at least 50m.

Some of the pockmarks in the southern area of the bay appear to be “eyed” that is, they exhibit a strong acoustic backscatter from the central bottom area even though the depressions do not penetrate entirely through the entire section of Holocene mud in which they form. This acoustic characteristic may be a consequence of acoustic focusing or result from a dense accumulation of benthic organisms or carbonate deposits. A few of the pockmarks, mainly in the southern area of the bay are buried or ancient.

Fig. 3 (High Res).jpg

Fig 3. Grey scale multibeam image of Passamaquoddy Bay pockmark areas.

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